1. Field of the Invention
The present invention relates to a liquefied-natural-gas (LNG) gasifier and a method of gasification.
2. Description of the Related Art
A liquefied natural gas (LNG), which is liquid methane, is a liquid having a boiling point of about −165° C. The LNG is transported from a liquefaction station by sea with an LNG ship provided with a cryogenic tank. The LNG is handled at on-shore LNG-receiving terminals (stations) near a port of various places to be supplied to customers.
Such LNG-receiving terminals are provided with, for example, an insulated tank to hold the LNG from a ship, a gasifier (vaporizer) to vaporize the LNG to convert into a natural gas (NG), in other words, a heat exchanger, and a controlling and measuring installation that regulates and measures an amount of the LNG supplied to pipeline of a customer.
To serve the LNG at an area without such LNG-receiving terminal, a floating-production-storage-and-offloading (FPSO) vessel is proposed. The FPSO is provided with a gasifier to convert the LNG into the NG at sea. The NG converted from the LNG at sea, for example, on a ship, is supplied through a pipeline to an on-shore pipeline for NG (Patent Literature 1).
An example of an LNG gasifier on a ship is shown in FIG. 4. As shown in FIG. 4, a conventional LNG gasifier includes a pipe 2 to feed the LNG from an LNG storage tank 1. An outer surface of a pipe 2a is brought into contact with a heating medium such as seawater 3. The pipe 2a is surrounded by a tubular shell 4. A seawater pump 5 is provided inside the tubular shell 4 to send the seawater 3 through the shell 4. A motor 6 is provided in a ship 7 to drive the seawater pump 5. The NG obtained by vaporization is collected in a collection tank 8 and sent to shore by a pipe 9.
Patent Literature 1: Japanese Patent Publication No. 2003-517545
In the conventional LNG gasifier disclosed in patent literature 1, the seawater pump 5 needs to be provided inside the tubular shell 4 to supply seawater, which is used as a heat source for vaporization. This requires provision of the motor 6 to drive the seawater pump 5 as well as maintenance of the seawater pump 5.
On the other hand, when, for example an open-rack-type LNG gasifier as shown in FIG. 5 that uses seawater for heat exchange is provided in the FPSO, seawater 103 is brought into a seawater trough 101 from a seawater supply port 102. The LNG passing through a heat exchanging tube 104 is vaporized by means of the seawater 103 overflowing from the seawater trough 101. Thus, the open rack type LNG gasifier needs a stable supply of seawater. However, a stable supply of seawater from the seawater trough 101 cannot be maintained due to swaying of the ship.
Providing the FPSO with another type of LNG gasifier such as an LNG gasifier that carries out heat exchange by supplying a gas from a burner to a water cistern requires maintenance of the burner and accompanying combustion facilities. Moreover, this results in a high fuel cost.
Providing the FPSO with still another type of LNG gasifier such as an LNG gasifier that carries out heat exchange by means of an intermediate heating medium requires use of combustible liquefied-petroleum gas (LPG) or chlorofluorocarbon substitute as the intermediate medium. This causes difficult handling, for example, in inspection and maintenance.